1. Field of the Invention
The present invention relates to a focusing apparatus of a telescopic lens system, and in particular, relates to a focusing apparatus of a telescopic lens system, which is appropriate to be used in combination with a camera having an autofocusing mechanism.
2. Description of the Prior Art
A photographing lens system, which is used in a camera having an autofocusing mechanism, includes a focusing mechanism which moves either the entire photographing lens system or a portion thereof in the optical axis direction by operating a lens moving mechanism constituted by a combination of a motor and gear trains, in accordance with focal-position information detected by an AF module.
Especially in a telescopic lens system, the optical system itself is bulky and heavy. Therefore some telescopic lens systems employ an inner-focusing mechanism in order to reduce the weight of a lens group to be moved for focusing, and reduce the load imposed on the motor and the gear trains of the lens moving mechanism.
An AF module employed in a conventional autofocusing mechanism is to detect the amount of defocus. Accordingly, a traveling distance of a lens group to be moved for focusing cannot be obtained directly from such an AF module. The lens moving mechanism and the AF module are therefore activated at the same time so that both the amount of defocus and an optimum traveling distance of a lens group to be moved for focusing are obtained.
However, the longer the focal length of the optical system becomes, the larger the change in the amount of defocus is with respect to the traveling distance of a lens group. There are some cases where the amount of defocus exceeds the detection-capability of the AF module, as a result. Accordingly, in a combination of a conventional telescopic lens system and an autofocusing mechanism, an optimum traveling distance of a lens group cannot be detected. Due to this reason, there are some cases where an in-focus state cannot be detected.
Furthermore, in order to miniaturize the autofocusing mechanism, it is necessary to also miniaturize the motor which drives the autofocusing mechanism; however, the driving torque of the motor is reduced accordingly. Consequently, a lens group which is moved in order to perform focusing also needs to be miniaturized. In particular, in a telescopic lens system in which the focal length of the optical system thereof is long, an inner-focusing mechanism which is designed to move a portion of the optical system has to be employed in order to reduce the weight of a lens group to be moved for focusing. However, an inner-focusing mechanism inevitably makes the optical system complicated, and an increase of production cost is caused.
The present invention is devised in order to eliminate the above-mentioned drawbacks in the prior art, and is to provide a telescopic lens system in which a manual focusing operation and an autofocusing mechanism are performed in an associated manner without utilizing complicated mechanisms and optical systems nor causing an increase of production costs.
As an aspect of the present invention, there is provided a focusing apparatus applied to a telescopic lens system including more than two lens groups. The focusing apparatus includes two focusing mechanisms which are operable independently from each other, and are adapted to vary both absolute and relative positions of two lens groups in the optical axis direction.
Due to this arrangement, the two focusing mechanisms can be used as a manual focusing mechanism and an autofocusing mechanism respectively, so that there is no need to provide a complicated mechanism such as a clutch and the like.
The focusing apparatus of a telescopic lens system according to the present invention can be applied, regardless of a lens arrangement, to a lens system in general in which more than two lens groups are operated as the focusing lens groups. More specifically, the focusing apparatus can be applied to a Petzval-type optical system including a positive powered (hereinafter, positive) first lens group constituted by a positive lens element and a negative powered (hereinafter, negative) lens element, and a positive second lens group constituted by a positive lens element and a negative lens element. A Petzval-type optical system is applied to a telescopic lens system and the like due to a feature that optimum optical performance can be attained with a smaller number of lens elements, though a Petzval-type optical system has a drawback of a narrower angle-of-view.
According to the present invention, in a telescopic lens system constituted by a Petzval-type optical system, the manual focusing mechanism roughly moves any one of the entire optical system, the positive first lens group and the positive second lens group; and further, the autofocusing mechanism, which is independent from the manual focusing mechanism, and is electrically activated, precisely moves either one of the first lens group and the second lens group in order to obtain an in-focus state of an object.
Furthermore, according to the above structure, the traveling distance of the positive first or second lens group by the autofocusing mechanism can be made more minute (very short distance), so that the detection-precision on defocus by the AF module can be enhanced.
Still further, according to the above structure, the traveling distance necessary for focusing is shared by the positive first lens group and the positive second lens group. The traveling distance of the first lens group or the second lens group is shorter compared with that of the first lens group or the second lens group in a conventional focusing mechanism in which only one of the first and second lens groups is moved for focusing. Consequently, the change in optical performance due to the change in the distance between the positive first lens group and the positive second lens group is made smaller, according to the present invention. In other words, a complicated optical system such as an inner-focusing mechanism is not required, and an optical system similar to a conventional Petzval-type optical system can be utilized.
More specifically, the focusing apparatus of a telescopic lens system according to the present invention includes the first lens group and the second lens group in this order from the object. The two focusing mechanisms move the first lens group and the second lens group independently and respectively in the optical axis direction.
In the above-mentioned structure, the first and second lens groups preferably satisfy the following condition:
0.4 less than |TI/TII| less than 2.5xe2x80x83xe2x80x83(1)
wherein
TI designates the traveling distance of the first lens group required for focusing from an infinite distance to a finite photographing distance, and
TII designates the traveling distance of the second lens group required for focusing from an infinite distance to a finite photographing distance.
The first lens group and the second lens group can further satisfy the following condition:
0.15 less than DI-II/f less than 0.7xe2x80x83xe2x80x83(2)
wherein
DI-II designates the distance between the first lens group and the second lens group when an object at an infinite distance is in an in-focus state; and
f designates the focal length of the entire telescopic lens system.
The first lens group and the second lens group are respectively constituted by a combination of a positive lens element and a negative lens element, and the focusing apparatus of a telescopic lens system preferably satisfies the following conditions:
|SCI/SC| less than 0.8xe2x80x83xe2x80x83(3)
20 less than xcexdIpxe2x88x92xcexdInxe2x80x83xe2x80x83(4)
wherein
SCI designates the total sum of the reciprocal of the product of the focal length and the Abbe number (1/(fi*xcexdi)) of each lens element in the first lens group; and
SC designates the total sum of the reciprocal of the product of the focal lengths and the Abbe number(1/(fi*xcexdi)) of each lens elements in the entire telescopic lens system.
xcexdIp designates the Abbe number of the positive lens element of the first lens group; and
xcexdIn designates the Abbe number of the negative lens element of the first lens group.
One of the two focusing mechanisms can include a manual focusing mechanism which moves one lens group in the optical axis direction via a manual operation; and the other thereof can include an electric autofocusing mechanism which is activated by an electric means, e.g., a motor, and moves the other lens group in the optical axis direction.
The manual focusing mechanism can be applied to the first lens group or the second lens group, and the autofocusing mechanism can be applied to the second lens group or the first lens group.
Alternatively, the focusing apparatus of a telescopic lens system according to the present invention includes the first lens group and the second lens group in this order from the object. One of the two focusing mechanisms moves the entire telescopic lens system in the optical axis direction; and the other thereof moves the first lens group in the optical axis direction.
The entire telescopic lens system and the first lens group preferably satisfy the following condition:
0.2 less than |TA/TI| less than 1.5xe2x80x83xe2x80x83(5)
wherein
TA designates the traveling distance of the entire telescopic lens system required for focusing from an infinite distance to a finite photographing distance; and
TI designates the traveling distance of the first lens group required for focusing from an infinite distance to a finite photographing distance.
Note that the focusing apparatus of a telescopic lens system in which one focusing mechanism moves the entire telescopic lens system, and the other moves the first lens group preferably satisfies conditions (6) to (8) corresponding to the above-explained conditions (2) to (4).
Moreover, the focusing apparatus of a telescopic lens system according to the present invention includes the first lens group and the second lens group in this order from the object. One of the two focusing mechanisms moves the entire telescopic lens system in, the optical axis direction; and the other of the two focusing mechanisms moves the second lens group in the optical axis direction.
The entire telescopic lens system and the second lens group preferably satisfy the following condition:
0.2 less than |TA/TII| less than 1.5xe2x80x83xe2x80x83(9)
wherein
TA designates the traveling distance of the entire telescopic lens system required for focusing from an infinite distance to a finite photographing distance; and
TII designates the traveling distance of the second lens group required for focusing from an infinite distance to a finite photographing distance.
Note that the focusing apparatus of a telescopic lens system in which one focusing mechanism moves the entire telescopic lens system, and the other moves the second lens group preferably satisfies conditions (10) to (12) corresponding to the above-explained conditions (2) to (4).
As explained, the focusing apparatus of a telescopic lens system according to the present invention can be applied, regardless of a lens arrangement, to a lens system in general. For example, in the case of a three-lens-group arrangement including a positive first lens group, a negative second lens group and a positive third lens group, in this order from the object, the two focusing mechanisms can move the positive first lens group and the positive third lens group respectively; or the two focusing mechanisms can move the positive first lens group and the negative second lens group respectively.
In the case of the telescopic lens system including the first lens group and the second lens group, in this order from the object, the first and second lens groups can be constituted by a positive lens group, and preferably satisfy the following condition:
0.2 less than |TA/TI| less than 0.8xe2x80x83xe2x80x83(13)
wherein
TA designates the traveling distance of the entire telescopic lens system required for focusing from an infinite distance to a finite photographing distance; and
TI designates the traveling distance of the first lens group required for focusing from an infinite distance to a finite photographing distance.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-301873 (filed on Sep. 28, 2001) which is expressly incorporated herein in its entirety.